Co-extrusion is useful for many applications, including inter-digitated pn junction lines, conductive gridlines for solar cells, electrodes for electrochemical devices, etc.
In order to meet the demand for low cost large-area semiconductors, micro-extrusion methods have been developed that include extruding a dopant bearing material (dopant ink) along with a sacrificial material (non-doping ink) onto the surface of a semiconductor substrate, and then heating the semiconductor substrate such that the dopant disposed in the dopant ink diffuses into the substrate to form the desired doped region or regions. In comparison to screen printing techniques, the extrusion of dopant material on the substrate provides superior control of the feature resolution of the doped regions, and facilitates deposition without contacting the substrate, thereby avoiding wafer breakage. Such fabrication techniques are disclosed, for example, in U.S. Patent Application No. 20080138456, which is incorporated herein by reference in its entirety.
In extrusion printing of lines of functional material (e.g., dopant ink or metal gridline material) on a substrate, it is necessary to control where the bead of dispensed material (e.g., dopant ink) goes once it leaves the printhead nozzle. Elastic instabilities, surface effects, substrate interactions and a variety of other influences can cause the bead to go in many undesired directions. The problem is usually solved by running the deposition (printhead) nozzles very close to the substrate so that the bead sticks to the substrate before it can wander off. Unfortunately, this causes the printhead to get contaminated with ink, and in a high speed (>100 mm/sec) production deposition apparatus with print heads containing dozens of nozzles and substrates with considerable thickness variation (>50 microns), it is not practical to print in close proximity.
What is needed is a micro extrusion printhead and associated apparatus for forming extruded material beads at a low cost that is acceptable to the solar cell industry and addresses the problems described above. In particular, what is needed is a printhead assembly that includes a mechanism for controlling the direction of the extruded bead so that it is biased downward onto the substrate, and away from the printhead.